It is known to make electrical laminates and other composites from a fibrous reinforcement and an epoxy-containing matrix resin. Examples of suitable processes usually contain the following steps:
(1) An epoxy-containing formulation is applied to or impregnated into a substrate by rolling, dipping, spraying, other known techniques and/or combinations thereof. The substrate is typically a woven or nonwoven fiber mat containing, for instance, glass fibers or paper.
(2) The impregnated substrate is “B-staged” by heating at a temperature sufficient to draw off solvent in the epoxy-containing formulation and optionally to partially cure the epoxy-containing formulation, so that the impregnated substrate can be handled easily. The “B-staging” step is usually carried out at a temperature of from 90° C. to 210° C. and for a time of from 1 minute to 15 minutes. The impregnated substrate that results from B-staging is called a “prepreg.” The temperature is most commonly 100° C. for composites and 130° C. to 200° C. for electrical laminates.
(3) One or more sheets of prepreg are stacked or laid up in alternating layers with one or more sheets of a conductive material, such as copper foil, if an electrical laminate is desired.
(4) The laid-up sheets are pressed at high temperature and pressure for a time sufficient to cure the resin and form a laminate. The temperature of this lamination step is usually between 100° C. and 230° C., and is most often between 165° C. and 200° C. The lamination step may also be carried out in two or more stages, such as a first stage between 100° C. and 150° C. and a second stage at between 165° C. and 190° C. The pressure is usually between 50 N/cm2 and 500 N/cm2. The lamination step is usually carried out for a time of from 1 minute to 200 minutes, and most often for 45 minutes to 90 minutes. The lamination step may optionally be carried out at higher temperatures for shorter times (such as in continuous lamination processes) or for longer times at lower temperatures (such as in low energy press processes).
Optionally, the resulting laminate, for example, a copper-clad laminate, may be post-treated by heating for a time at high temperature and ambient pressure. The temperature of post-treatment is usually between 120° C. and 250° C. The post-treatment time usually is between 30 minutes and 12 hours.
It is conventional in the preparation of epoxy-containing laminates to incorporate into an epoxy resin composition a hardener (also referred to as a “curing agent” or a “crosslinking agent”) in order to provide the crosslinking of the epoxy composition to form a thermoset resin. Various hardeners for epoxy resins are generally known including amines, phenolics, anhydrides, carboxylic acids, mercaptans and isocyanates. Epoxy resins can also homopolymerize by reacting with both nucleophilic and electrophilic species.
The current trend of the electrical laminates industry requires materials with improved dielectric properties including lower dielectric constant (Dk) and loss factor (Df); superior thermal properties including high glass transition temperature (Tg) and decomposition temperature (Td); and good processability. One known approach for improving laminate properties consists of curing a flame retardant epoxy resin with an anhydride hardener such as styrene-maleic anhydride copolymer (SMA). For example, the use of SMA as cross-linking agents for epoxy resins is described in U.S. Patent Application Publication No. 2002/0082350 A1. The main drawback of this known SMA hardener system is that it provides laminates with high brittleness. The known SMA hardener system also has poor processability because a prepreg powder made with the known SMA hardener is easy to remove from a substitute such as a glass web, creating a lot of dust when the prepreg is handled (a so-called “mushroom” effect). In addition, the resulting laminate made from the known SMA hardener system has a low toughness and is easy to delaminate, creating defects during drilling operations of the laminates. Often, the resulting prepreg also shows poor prepreg cosmetics due to entrapped gas bubbles.
It is therefore desired to provide a hardener system for epoxy resins without the disadvantages of the prior art.